Age-related changes in liver drug-metabolizing enzymes

Age-related changes in liver structure and function: Implications for disease ?

The geriatric populations of many countries are growing rapidly and they present major problems to healthcare infrastructures from both medical and economic perspectives. The elderly are predisposed to a variety of diseases, which contribute to a marked increase in morbidity in this subpopulation. The incidence of liver disease increases in the elderly, but the cellular and subcellular perturbations that underlie this suspected predisposition to pathology remain unresolved. Several age-related changes have been documented, including (a) a decline in liver volume, (b) an increase in the hepatic dense body compartment (lipofuscin), (c) moderate declines in the Phase I metabolism of certain drugs, (d) shifts in the expression of a variety of proteins and (e) diminished hepatobiliary functions. Other more subtle changes (e.g., muted responses to oxidative stress, reduced expression of growth regulatory genes, diminished rates of DNA repair, telomere shortening) may contribute to reduced hepatic regenerative capacity, shorter post-liver transplant survival and increased susceptibility to certain liver diseases in the elderly.

Age related changes in NADPH cytochrome c reductase activity in mouse skin and liver microsomes

In this study, the effect of ageing on the activity of cutaneous and hepatic NADPH cytochrome c reductase, a component of the mixed function mono-oxygenase system, was studied in young adult and senescent male C57B1/6J mice. Horse heart cytochrome c was used as substrate and its reduction by skin and liver microsomes was measured spectrophotometrically. Statistical analysis of our results show a significant decrease in cutaneous enzyme activity with age. This, however, was not the case in liver where no significant difference was seen, consistent with previous studies on mixed function mono-oxygenase activity in mouse liver. This suggests that the ageing process may have a tissue specific effect on the monooxygenase system in the C57B1/6J strain of mouse. However, further work in other rodent species and humans is required to fully characterize the effect of ageing on the drug metabolizing capacity of skin as compared to liver.

Age-related susceptibility to hepatotoxicants

Limited information is available regarding age-associated events that lead to differences in vulnerability to chemicals that injure the liver. For some agents, such as allyl alcohol, alterations in metabolic activation, by liver biotransformation enzymes, are responsible for age-associated changes in severity of liver damage. For other toxicants, such as carbon tetrachloride, there appears to be no relation between changes in activation/detoxification processes and the effects of aging on the extent of liver injury. With diquat, a rise in iron content seems to explain the increased toxicity observed in hepatocytes of old rats compared with those of young-adult rats. Additional research is needed to identify the mechanisms responsible for age-dependent differences in sensitivity to environmental chemicals.

Effect of aging on mixed-function oxidation and conjugation by isolated perfused rat livers

Aging is known to decrease hepatic cytochrome P450 content in rats. However, limited information is available on the effects of aging on mixed-function oxidation and conjugation in intact liver. The purpose of these studies was to determine the effects of aging on oxidation and conjugation of p-nitrophenol (pNP) in perfused livers from male Sprague-Dawley rats. Livers from senescent (22-24 months) or young adult (3-6 months) rats were perfused in a nonrecirculating hemoglobin-free system and supplemented with pNP (60 microM). Glucuronide and sulfate conjugates of the oxidation product, 4-nitrocatechol, in effluent perfusate were cleaved enzymatically and 4-nitrocatechol was determined colorimetrically. Rates of 4-nitrocatechol production were decreased in senescent compared with young adult rats (0.67 +/- 0.14 vs 0.92 +/- 0.15 micromol/g/hr). However, the rates of oxidation of pNP in microsomes from senescent rats were similar to those in young adult rats. Hepatic malate content was decreased approximately 50% in livers from senescent compared with young adult rats in the presence and absence of pNP, suggesting that movement of reducing equivalents from the mitochondria to the cytosol, and thus cytosolic NADPH supply, may have been diminished by senescence. The rates of conjugation of 60 microM pNP in perfused livers from senescent rats were similar to those in young adult rats, but Km and Vmax values of microsomal 4-nitrocatechol glucuronyltransferase were about 2.5- and 1.6-fold higher, respectively, in livers from senescent compared with young adult rats. Hepatic glycogen content was about 50% lower in livers from senescent compared with young adult rats, but the contents of UDP-glucose and UDP glucuronic acid were similar between the two groups. Taken together, the data are consistent with the hypothesis that rates of mixed-function oxidation are decreased in intact livers from senescent compared with young adult rats, due possibly to age-related changes in cofactor supplies. Glucuronidation of low, but not high, concentrations of substrates may be affected by age-related changes in Km and Vmax values of microsomal glucuronyltransferase.

The effects of age on the pharmacokinetics and biotransformation of theophylline in vivo and in vitro in the Mongolian gerbil (Meriones unguiculatus)

The effect of post maturational aging on the in vivo disposition of theophylline was examined in the Mongolian gerbils (Meriones unguiculatus) aged 30-39 (old), 12-18 (middle-aged) and 3 (young) months following a 20 mg/kg i.p. dose. Biotransformation of theophylline was also examined in liver microsomes from non-induced and 3-methylcholanthrene induced gerbils. Analysis of theophylline plasma kinetics showed decreased clearance, increased half-life and increased volume of distribution in old vs. young animals. Clearance to the 1,3-dimethyluric acid metabolite was similar for all age groups, while clearance to the 1-methyluric acid metabolite was significantly lower in the middle-aged group compared to that of young and old gerbils. Urinary recovery of 1-methylurate was increased in old vs. young and middle-aged animals while recovery of theophylline was decreased. 3-Methylcholanthrene induction resulted in decreased recovery of theophylline and increased recovery of 1,3-dimethylurate and 1-methylurate in young and middle-aged gerbils compared to non-induced controls. Decreased microsomal protein content was observed in old vs. young and middle-aged gerbils and an age-related decrease in cytochrome P-450 content (nmol P-450/g liver) was also observed. The rate of dimethylurate formation was decreased 37% in microsomes from old vs. young and middle-aged gerbils. 3-Methylcholanthrene administration resulted in a 2- and 1.5-fold increase in the rate of 1,3-dimethylurate formation in young and middle-aged gerbils, respectively. The results of these experiments indicate that the Mongolian gerbil may be useful for the study of the biochemical mechanisms underlying age-related changes in the biotransformation and kinetics of theophylline.

Changes in the male rat hepatic cytochrome P-450 level, heme oxygenase and delta-aminolevulinic acid synthase activities at various stages of life

delta-Aminolevulinic acid synthase and heme oxygenase are closely functionally connected with the hepatic microsomal mixed function oxydase system. There is a wide range of changes in microsomal heme oxygenase activity, cytochrome P-450 level and mitochondrial delta-aminolevulinic acid synthase activity in rat livers at various stages of life. We tried to investigate the spontaneous phases of these enzymatic protein activities and the levels of cytochrome P-450 as a function of age. We noted that each of these proteins change their activity and level very characteristically during the rats' lifetime. However, correlations between heme oxygenase activity, delta-aminolevulinic acid synthase activity and cytochrome P-450 levels still need to be investigated to elucidate the mechanism of these connections.

Ongoing hepatocellular regeneration and resiliency toward galactosamine hepatotoxicity

In previous studies, we reported that the age-dependent hepatotoxicity of galactosamine (GalN) was evident in hepatocytes maintained in primary cultures. Cellular proliferation and tissue repair are not manifested in response to injury in this in vitro system. Neonatal (5-day) rats have ongoing hepatocellular proliferation in contrast to adult (5-month) rats, and should be therefore resilient to GalN toxicity. Liver injury was assessed by serum transaminases (ALT, AST), 3H-thymidine (3H-T) incorporation into nuclear DNA, and content of hepatocellular nuclear DNA. While the dose of 400 mg/kg did not cause any significant liver injury in the neonates, it did produce significant liver injury in adult rats. At a dose of 800 mg/kg, GalN produced significant injury in the neonates. Because 400 mg/kg causes clearly demonstrable liver injury in the adult and no injury in the neonates, this dose was used for further studies. In addition to the above measures of injury, uracil nucleotides (UTP, UDP, and UMP), glycogen, histopathology, and autoradiographic examination of liver sections were used to assess the liver injury in neonatal and adult rats. In a time-course study, all of the above were measured at 0, 12, 24, 36, 48 and 72 h after GalN administration. Serum enzyme elevations as well as the appearance of necrotic and swollen hepatocytes were maximal at 24 h in the adults rats. In contrast to these observations in the adult rats, none of these measurements indicated significant liver injury in the neonates. 3H-T incorporation into nuclear DNA was much higher in the neonatal liver in comparison to the adults reflecting the difference in regeneration. Hepatocellular nuclear DNA was also higher in the neonate and was significantly decreased due to GalN treatment. In the adult rats, the quiescent normal level of 3H-T incorporation and nuclear DNA content were further decreased at 12 h, increased at 48 h and returned to normal low, quiescent levels at 72 h. In the neonates mitotic activity of hepatocytes was higher than in the adult rats. In the adult rats, mitotic activity was increased at 48 h after GalN administration and returned to normal at 72 h. In the neonates GalN did not alter the mitotic activity significantly. These findings demonstrate that in the presence of hepatocellular regeneration, galactosamine toxicity is minimal while in the absence of it, clear toxicity is manifested. In conclusion, while perturbation in uracil nucleotides and related biochemical events may explain the infliction of liver injury by GalN in an age-dependent fashion, the extent of tissue repair impacts decisively on the final outcome of injury.

Studies on the age-dependent effects of galactosamine in primary rat hepatocyte cultures

Galactosamine (GalN) has been known to induce liver injury by depletion of uracil nucleotides. The objective of the present work was to examine age-dependent toxicity of GalN in primary hepatocyte cultures. Hepatocytes from fetal (Day 20 of gestation), neonatal (2.5-day), adult (5-month), and aged (30-month) rats were established as monolayered cultures. LDH leakage, cell viability, UTP, UDP, and UMP were measured as end points of toxicity in cultures exposed to 5 mM GalN. LDH leakage was increased and cell viability was decreased in adult rat hepatocytes at 48 and 60 hr after treatment. Although similar effects were observed in hepatocytes from aged rats, these cells appeared resilient to GalN toxicity as indicated by significantly less LDH leakage and cell death. Fetal and neonatal rat hepatocytes also exhibited greater resiliency to GalN based on the same end points. The UTP, UDP, and UMP levels of aged hepatocytes (30-month) were higher than control adult levels to begin with and dropped after GalN treatment. The level of UMP at 60 hr was similar to that of normal adult cells, but the UTP and UDP levels were significantly higher in aged hepatocytes in comparison to those of adult hepatocytes. The levels of uracil nucleotides in the fetal and neonatal cells were the same as those in adult cells, but did not decrease significantly after exposure to GalN. These findings show that aged rat hepatocytes have a higher set point for uracil nucleotides, which is consistent with the relative resiliency of these cells to GalN injury. Neonatal and fetal cells have the same set point for these nucleotides as adult rats, but are relatively resistant to GalN-induced depletion. In conclusion, the differences in toxicity of GalN may reside in age-related differences in the regulation of uracil nucleotide biochemistry.

Cytochrome P-450 isozyme pattern is related to individual susceptibility to diethylnitrosamine-induced liver cancer in rats

Differences in susceptibility to chemical carcinogenesis between rodent strains and species have been linked to variations in genetically-determined mixed function oxidase activities. In order to verify whether such variations also determine the susceptibility of individual animals of the same strain to a chemical carcinogen, outbred male Wistar rats were administered diethylnitrosamine (DEN) (1, 2, or 3 mg/kg) five times a week for 20 weeks. The relationship was examined between the outcome (i.e., presence or absence of liver tumors, and latency period) and the hepatic activities of mixed function oxidases and conjugating enzymes, as well as of O6-methylguanine-DNA-methyltransferase, measured before the carcinogen treatment. In addition, the metabolic profiles of two model drugs, antipyrine and disopyramide, in the urine were analyzed and correlated with the carcinogen susceptibility. The length of the latency period of hepatocellular tumors in individual rats was negatively related to the activities of hepatic dimethylnitrosamine N-demethylase, aryl hydrocarbon hydroxylase and epoxide hydrolase and positively related to the amount of microsomal protein. Consistent relationships between the other 10 measured parameters and the susceptibility to DEN-induced carcinogenesis were not detected. Long-term treatment with DEN slightly decreased the proportion of metabolism of antipyrine into norantipyrine, and increased the share of 4-hydroxyantipyrine; a decrease in the metabolism of disopyramide to N-deisopropyldisopyramide was also detected. It is concluded that the pattern of cytochrome P-450 isoenzymes is related to differences in individual susceptibility to nitrosamine-induced carcinogenesis. The relationship was most marked at low dose levels, which are the levels at which nitrosamine exposures of humans are known to occur.

Aberration of heme and hemoprotein in aged female rats

The magnitude and duration of drug action is determined partially by the activity of the drug metabolizing enzyme systems in the liver. The pharmacological effectiveness of many drugs is altered during the aging process. In this study, the regulation of heme metabolism and hemoprotein content was examined in livers of aged female rats. The activities of hexobarbital hydroxylase and aniline hydroxylase, indicators of mono-oxygenase function, were decreased in aged rats by 31% and 24%, respectively, as compared to values in young rats. This was accompanied by a proportional decrease in the level of cytochrome P-450 (26%). Additionally, the activity of delta-aminolevulinic acid synthetase (ALA-S), the rate-limiting enzyme in heme synthesis, and the microsomal concentration of heme were also decreased by 33% and 26%, respectively, in these animals. In contrast, the basal activity of microsomal heme oxygenase (MHO), the rate-limiting enzyme in heme degradation, and the percent heme saturation of tryptophan pyrrolase (TPO), a sensitive indicator of changes in the availability of heme in the "regulatory" heme pool, were increased by (87%) and (31%), respectively, in the aged rats. The serum concentration of bilirubin, an indicator of erythrocyte breakdown and/or liver function was likewise increased in these animals. In view of these findings, we suggest that the high activity of MHO and the low level of ALA-S may be a significant causative factor for the decreased microsomal concentration of heme, cytochrome-P-450 and its dependent monooxygenase activities in senescent female rats.

Low level of lipid peroxidation in newborn rats. Possible factors for resistance in hepatic microsomes

Hepatic rough and smooth microsomes of newborn rats show less sensitivity to ascorbate- and NADPH-induced lipid peroxidation as compared to those of adult rats. Though optimum concentrations of Fe2+, ascorbate and Fe3+ significantly increase lipid peroxidation in both age groups, the lipid peroxidation observed in newborns is much less compared with that of adults. Microsomal fractions from newborn rats contain significantly lower amounts of phospholipid, NADPH cytochrome c reductase, cytochrome P-450 and a lower degree of unsaturation in lipids. These fractions also exhibit high cholesterol:phospholipid ratios. The resistance to lipid peroxidation observed in the newborns appears to be due to the low availability of substrate and high cholesterol:phospholipid ratio.

Effects of development and ageing on pulmonary NADPH-cytochrome c reductase, glutathione peroxidase, glutathione reductase and thioredoxin reductase activities in male and female rats

The behaviour of the principal NADPH-consuming detoxification enzymes (NADPH-cytochrome c reductase, glutathione peroxidase-glutathione reductase system, and thioredoxin reductase) was studied during development and senescence of the rat lung. We have also studied the influence of sex on the development and senescent values. The NADPH-cytochrome c reductase activity increases at birth and afterwards remains constant until the 25th day after birth, at which age there is a maximum activity. Its activity decreases during the ageing period in both sexes. The glutathione reductase and thioredoxin reductase activities show significant differences with respect to sex during the adult stage, however during ageing these differences disappear. These enzymes show maximum activity at 25 days after birth, and afterwards the activity decreases continuously until the adult levels are reached. The activity of glutathione reductase is increased during the ageing period, especially in the female rats, however, in senescence the levels of thioredoxin reductase are lower than in the adult stage. The glutathione peroxidase shows a significant difference between both sexes during senescence and in the male its activity in this stage is higher than during development and adulthood.

Comparison of the hepatic mixed function oxidase system of young, adult, and old non-human primates (Macaca nemestrina)

The influence of age on the mixed function oxidase system from a non-human primate was studied. Microsomes were isolated from the livers of female Macaca nemestrina ranging from 2 to 21 years of age. No significant age-related change was observed in either the cytochrome P-450 content or the NADPH cytochrome c reductase activity. In addition, the ability of the microsomes to metabolize benzo [a]pyrene did not change significantly with age. These observations contradict studies with liver tissue from laboratory rodents in which an age-related decline in the mixed function oxidase system is generally observed. The lipid composition of the liver microsomes was studied also. Both the cholesterol and total phospholipid content of the liver microsomes increased significantly with age; however, the ratio of cholesterol to phospholipid remained constant. The percentage of individual phospholipids in the microsomes changed only slightly with age. These results provide new information on the effect of age on the mixed function oxidase system and indicate that one must be cautious in extrapolating from studies with liver tissue from laboratory rodents to primates.

Age-dependent alterations in rat ventricular myocardium: a quantitative analysis

A quantitative electron microscopic examination of the ventricular myocardium of the male Fischer 344 rat was undertaken to determine the extent of age-dependent changes in ultrastructure. Rats of 6, 16 and 30 months of age were examined using a non-biased stereological test system. Volume fractions of mitochondria, myofibrillar mass and lipid remained unchanged during aging. Dense body volume fraction increased four-fold over the period studied, while the sarcoplasmic reticulum volume increased only from 6 months to 16 months and did not change thereafter. Biochemical analysis of the specific activities of the two lysosomal enzymes, acid phosphatase and beta-glucuronidase demonstrated a differential response to aging, with acid phosphatase remaining constant and beta-glucuronidase increasing slightly. Neither enzyme activity correlated with the pronounced change in dense body volume fraction. This study provides clear evidence that the previously observed changes in cardiac performance associated with aging are not readily explicable on the basis of a degradation of cardiac ultrastructure.

Age-related differences in the response of hepatic oxygen consumption to thermal stress in the male garden lizard

The main aim of the present study is to compare the thermal adaptation capacity of young, middle-aged and old lizards with respect to hepatic oxygen consumption. Cold exposure (1 h at 0-4 degrees C) caused a decline in endogenous oxygen consumption in young but did not evoke significant changes in middle-aged and old lizards. On the other hand, with 5 mM succinate the rate of respiration increased marginally only in cold-exposed old lizards. This suggests that the metabolic response to cold is age-dependent in these lizards. Exposure of homogenates at 40 +/- 1 degree C for 15 min led to a rise in the hepatic oxygen consumption of young but not in that of either middle-aged or old lizards. There were no changes in the rate of oxygen consumption upon exposure for a similar period at 45 +/- 1 degree C of liver homogenates from lizards of three different age groups. However, exposure at 51 +/- 1 degree C resulted in a higher degree of decline in the rate of hepatic oxygen consumption in middle-aged and old than in young lizards. Thus, the young lizards appear to be better-adapted to hyperthermic stress than the middle-aged and old.

Effects of age and obesity on the metabolism of lindane by black a/a, yellow Avy/a, and pseudoagouti Avy/a phenotypes of (YS x VY) F1 hybrid mice

Lindane (gamma-hexachlorocyclohexane) has been shown to produce hepatomas in some strains of mice but not in others. Genetic factors and/or altered metabolism may play a role in the susceptibility to lindane-induced hepatomas. This study reports the effect of age and obesity on the comparative metabolism and disposition of lindane in obese yellow Avy/a and in lean pseudoagouti Avy/a and black a/a phenotypes of (YS x VY) F1 hybrid female mice at 8, 17, 30, 56, and 86 wk of age. At 24 h prior to sacrifice the mice were dosed p.o. with 18 mg lindane (containing 55 microCi [U-14C]lindane/kg). Aging altered the biotransformation of lindane such that while the excretion of lindane and its metabolites declined, the proportion of conjugated and polar metabolites increased. Tissue storage was elevated in older animals. In the yellow Avy/a mice, which are known to have a predisposition to the formation of hepatomas, there was accelerated and prolonged growth, reduced metabolite excretion, a greater proportion of conjugated metabolites, and higher dechlorinase activity compared to that of their pseudoagouti Avy/a and black a/a siblings.

Effect of cimetidine on delta-aminolevulinic acid synthase and microsomal heme oxygenase in rat liver

Cimetidine is a well known inhibitor of the heme-containing enzyme cytochrome P-450. We have found that it also inhibits delta-aminolevulinic acid synthase (ALA-S) and microsomal heme oxygenase, the rate-limiting enzymes for heme synthesis and heme degradation respectively. Cytochrome P-450 content was decreased but microsomal heme concentration remained unaltered for a period of 30 min after in vivo cimetidine administration to rats. In vitro incubation of cimetidine with each of the above enzymes revealed no direct effect of cimetidine on ALA-S but about 50% inhibition of heme oxygenase and 20% reduction in cytochrome P-450 content. This suggests that a metabolite of cimetidine inhibits ALA-S activity in vivo, while the drug itself or a metabolite inhibits heme oxygenase both in vivo and in vitro. A rise in ALA-S activity seen after its early inhibition and its return to approximate control values after 60 min suggest a reversible inhibition of ALA-S by a metabolite of cimetidine and may correspond to its clearance from the animal. An elevation in microsomal heme content paralleled the rise in ALA-S activity while microsomal heme oxygenase activity returned to only 65% of control value 60 min after cimetidine treatment. Cytochrome P-450 content did not change after its initial decrease, suggesting that irreversible alteration had occurred.

Influence of aging on the susceptibility of rats to hepatotoxic injury

The effects of allyl alcohol, galactosamine, bromobenzene, and corn oil administration were evaluated in male Fischer 344 rats at 4 to 5, 14 to 15, and 24 to 25 months of age to determine if susceptibility to hepatotoxic injury is modified as a consequence of aging. Parameters measured were (1) severity of hepatocellular necrosis as judged by light microscopy of liver sections, (2) activity of alanine aminotransferase and aspartate aminotransferase in serum, and (3) hepatic microsomal cytochrome P-450 content and NADPH-cytochrome P-450 reductase activity. Allyl alcohol toxicity was more severe in middle-aged and old rats than in young-adult rats. In contrast, galactosamine and bromobenzene toxicities were slightly decreased or unchanged in old rats. The results demonstrate that aging has effects on some types of chemically induced hepatotoxicity.

Effect of aging on liver glutathione levels and hepatocellular injury from carbon tetrachloride, allyl alcohol or galactosamine

Severity of liver damage 24 hr after i.p. administration of carbon tetrachloride (0.2 ml/kg), allyl alcohol (0.036 ml/kg) or galactosamine (400 mg/kg) was evaluated in male rats at 4-5, 14-15 or 24-25 months of age. Allyl alcohol hepatotoxicity, as judged by light microscopy and serum alanine aminotransferase levels, increased markedly as a function of age. In contrast, carbon tetrachloride and galactosamine toxicities were unchanged or slightly diminished in old rats. Hepatic glutathione (GSH) concentrations were unaffected by aging; thus, the age-dependent increase in susceptibility to allyl alcohol toxicity was not a result of diminished GSH availability in old age. Hepatotoxicant-induced changes in GSH were observed in allyl alcohol-treated old rats (20% increase) and in galactosamine-treated young-adult and middle-aged rats (30% decrease).

Modification of age-induced changes in heme and hemoproteins by testosterone in male rats

Age-related changes in heme and hemoproteins, as well as the effect of testosterone treatment on these modifications were examined in male Sprague-Dawley rats. The activity of delta-aminolevulinic acid synthase (ALA-S) and the microsomal concentration of heme in aged rats were decreased by 37% and 33%, respectively, as compared to young values. In contrast, a marked increase in the activity of microsomal heme oxygenase (MHO) was seen in these animals. In aged rats, the level of cytochrome P-450 was decreased by 37%, as compared to young values. Furthermore, the activities of benzo[a]pyrene hydroxylase and aniline hydroxylase were decreased in proportion to the microsomal content of cytocyrome P-450. Steroid delta 4-hydrogenase, an index of endogenous substrate metabolism, exhibited no changes in activity during the aging process. The level of various hemoproteins such as cytochrome b5 and tryptophan pyrrolase in aged animals remained unaltered despite the decreased hepatic concentration of heme. It is worth noting that testosterone treatment of aged castrated rats restored the level of heme and cytochrome P-450 and the altered enzymatic activities of ALA-S and MHO to the "young" condition. In view of these findings, it is concluded that the events which lead to the low level of heme and cytochrome P-450 and its dependent mixed function oxidase activity during the senescent period could be due to increased rates of MHO and diminished ALA-S activities in these animals.

Effects of dietary protein level on hepatic microsomal mixed-function oxidase systems during aging in two generations of Syrian hamsters

Syrian hamsters were fed one of three levels of lactalbumin (10, 20, or 40%) from 4 weeks of age. Parents (F0 generation) and their offspring (F1 generation) remained for life on the parental diet. Randomly selected hamsters from both generations were killed at 0, 6, 18, 30, and 60 weeks to evaluate hepatic microsomal drug metabolism systems. Microsomal protein content rose with increased dietary protein in both generations of females at 30 and 60 weeks and in both generations of males at 18 weeks. Microsomal protein rose during life, but the timing and magnitude of the increase varied with sex, diet, and generation. Cytochrome P-450 content increased with elevation in dietary protein at most measurement times in females and at all measurement times in F1 generation males. Hepatic aryl hydrocarbon hydroxylase (AHH) and aniline hydroxylase (ANH) activities were influenced by dietary protein, primarily at 6, 18, and 30 weeks, and these values tended to decline between maturity and senescence. AHH generally increased as dietary protein rose, except at 30 weeks, at which time the group fed the medium protein level usually had the highest activity. ANH activity was either elevated in groups fed the 20 or 40% lactalbumin levels or depressed in these groups at 6 and 18 weeks when these values were influenced by dietary protein. The effects depended upon sex and generation. Age effects on hepatic microsomal metabolism in hamsters varied by sex and dietary protein and were somewhat different than those reported for other rodents.

Changes in enzymes of hepatic rough and smooth microsomes during postnatal development and ageing of rats

Significant changes are observed in wet weight, microsomal protein content and enzymes of purified rough and smooth microsomes of liver during postnatal development and ageing of female Wistar rats. Protein content of total microsomes increases up to 15 days of age and remains steady during subsequent development, unlike that of rough and smooth microsomes which shows changes throughout the same period. Activities of cytochrome P-450, cytochrome b5 and NADPH-cytochrome c reductase increase during the period of maturation and decline during senescence. The decrease during senescence is at different rates in the two microsomal fractions. Microsomal glucose-6-phosphatase, but not adenosine triphosphatase, shows a similar increase during development and decrease during senescence.

Age-dependent changes in rat liver microsomal NADPH cytochrome C (P-450) reductase: a kinetic analysis

Rat liver microsomal NADPH cytochrome c (P-450) reductase (EC 1.6.2.4) exhibits several marked age-dependent changes, including a decline in specific activity, reduced inducibility and enhanced thermostability, which are indicative of an alteration in the quality of this enzyme. The present study examined the kinetic profile of the microsomal-bound enzyme in an effort to further define the effects of aging on the hepatic mixed function oxidase system. Intact microsomes isolated from young adult (3 months), mature (16 months) and senescent (27 months) rats were subjected to an extensive double reciprocal kinetic analysis employing NADPH and cytochrome c as substrates. The Km values obtained with the initial substrate (NADPH) remained unchanged with animal age, whereas there was a decline in this parameter for the artificial acceptor substrate, cytochrome c. The Vmax values for both substrates were reduced as a function of increasing age, perhaps reflecting a concomitant decline in the relative amount(s) of efficient reductase in the microsomes.

Differential effects of aging on hepatic microsomal monooxygenase induction by phenobarbital and beta-naphthoflavone

The influence of aging on hepatic microsomal monooxygenase induction by phenobarbital (PB) or beta-naphthoflavone (BNF) was investigated in male Fischer 344 rats maintained in a constant environment. PB-induced increases in microsomal cytochrome P-450 content and NADPH-cytochrome c reductase activity were similar in rats aged 3-5 months (young-adult) and 24-25 months (old), but increased in benzephetamine N-demethylase activity were markedly diminished in the old rats. Separation of hepatic microsomal proteins by sodium dodecylsulfate gel electrophoresis demonstrated that aging decreased the induction by PB of a polypeptide with a molecular weight of 52,500. BNF-induced increases in microsomal cytochrome P-450 and nitroanisole O-demethylase activity were greater in old than in young-adult rats, and BNF induction of 55,000 and 57,000 molecular weight microsomal polypeptides was increased slightly in livers from old rats. The results indicate that age-related effects on monooxygenase induction vary with different inducers of the hepatic microsomal enzyme system.

The effect of age on the biliary excretion of digitoxin and its metabolites in female BN/Bi rats

The biliary excretion of digitoxin (Dt3) and its metabolites was studied in female BN/Bi rats of different ages ranging from 3 to 30 mth. The disappearance of radioactivity from plasma after an i.v. injection of [3H]Dt3 (0.01 mg/100 g body weight) gradually slowed as rat age advanced. The 2-h biliary recovery of radioactivity (percent of the dose) also showed an age-dependent decrease resulting in a 43% decrease at 30 mth compared to the 3-mth-old value. The amount of unchanged Dt3 in the bile, which was about 20% of the total activity in the bile of 3-mth-old rats, did not decrease with age, while the excretion of various Dt3 metabolites all decreased with age. It is suggested that the capacity of the liver to metabolize Dt3 is decreased with age in female BN/Bi rats, which may cause an alteration in the Dt3 pharmacokinetics with age.

Age-related changes in mutagen activation by rat tissues

Age-related changes in drug metabolism of the liver, lung and kidney of adult female Long-Evans rats were determined by measuring changes in mutagen formation. Activation of aflatoxin B1 (AFB1), 2-aminofluorene (AF) and 2-acetylaminofluorene (AAF) to mutagenic derivatives was assayed using the Ames Salmonella test system. The promutagens were incubated with tissue fractions from rats ranging in age from 2.5 to 25 months. With all three compounds, hepatic, renal and pulmonary activation was lower in the senescent than in the young adult animals. The largest decrease, however, occurred prior to middle-age, i.e. before 9-13 months. In liver and kidney, little change was detectable between the middle-aged and the old (20-25 months) animals. However, pulmonary metabolism in the oldest animals was slightly higher than in the extracts from the middle-aged rats. The observed decline in mutagen activation may thus be a function of maturation rather than senescence.

Substrate specificity of age-related changes in the inducibility of hepatic microsomal monooxygenases in middle-aged rats

Hepatic microsomal monooxygenase induction was investigated in young-adult and middle-aged male Fischer 344 rats. Monooxygenase components and drug metabolism activities were determined in liver microsomes prepared from rats treated with phenobarbital (PB, beta-naphthoflavone (BNF) or methyltestosterone (MT) and compared with values from untreated rats. PB and BNF effects on cytochrome P-450 concentration and cytochrome c reductase activity were similar in young-adult and middle-aged animals. However, the extent of cytochrome P-450 induction by MT was less in the older animals. The age-related changes in induction of drug metabolism activities differed with different substrates for the monooxygenase system. In contrast to the inducibility of benzphetamine N-demethylation and aniline hydroxylation, which was diminished in the older rats, the inducibility of nitroanisole O-demethylation was enhanced. The results imply that qualitative changes in the microsomal enzyme system occurred as the animals progress from young to middle adulthood.